This invention relates generally to non-dispersive infrared spectrophotometers. More particularly, this invention relates to an improved gas analyzer which measures the concentration of one or more gases in a gas mixture present in a sample cell.
Gas analyzers of the non-dispersive type typically operate on the premise that the concentration of a designated gas can be measured: (1) by passing a beam of infrared radiation through the gas, and (2) ascertaining the attenuation of the energy in a narrow wavelength band absorbable by the designated gas with a detector capable of generating an electrical output signal proportioned to the energy in the band passing through the gas. Examples of such analyzers are disclosed in U.S. Pat. Nos. 4,346,296, 4,423,739, and 5,811,812.
NDIR gas analyzers utilize an IR source, typically an electric heater, to provide IR radiation through a gas sample contained in a sample cell for detection by a detector. The amplitude of the signal detected by the detector at a wavelength, which corresponds to the absorption wavelength of a gas of interest, provides an indication of the concentration of that gas in the sample. Concentration of gases, such as CO, CO2, hydrocarbons (HCs), anesthetic agent gases, exhaust gases, Freon, or other gases can be determined by IR radiation spectroscopes. Each species of gas typically has one or more distinct IR absorption characteristics and better absorbs IR radiation at or near a particular wavelength. The absorption of IR radiation at a frequency corresponding to a characteristic absorption wavelength of a particular gas species decreases, as the concentration of that species in the gas sample. In other words, the amplitude of the signal detected by the IR detector at a wavelength corresponding to a characteristic absorption wavelength of a particular gas species is inversely proportional to the concentration of that species in the gas sample.
Medical applications of these gas analyzers include the monitoring of end-tidal carbon dioxide, i.e., the concentration of carbon dioxide in a patient""s exhalations. This expired carbon dioxide level can be employed by medical personnel to monitor the operation of a ventilator to assist a patient""s breathing. A sample cell is designed to be inserted into the airway of a patient on a ventilator and includes a pair of opposed windows having a line of sight positioned so as to allow a beam of infrared radiation to pass therethrough. The sample cell confines the expired gases to a flow path with a precise, transverse dimension, and it furnishes an optical path between an infrared radiation emitter or source, and an infrared radiation detector assembly having a plurality of detectors electrically responsive to impinging radiation.
The infrared radiation traverses the gases in the sample cell where it is attenuated because part of the radiation is absorbed by the designated gas or gases being analyzed. The attenuated beam of infrared radiation is then filtered to eliminate energy at wavelengths lying outside the narrow band absorbed by a particular gas being measured. The infrared radiation in that band impinges upon a detector which consequently generates an electrical signal proportional in magnitude to the intensity of the infrared radiation impinging upon it.
Typically, a narrow band optical filter is positioned in front of the detector to pass a narrow band of only those wavelengths of infrared radiation absorbed by the gas or gases of interest. For example, a narrow band filter with a center wavelength of approximately 4.3 microns is conventionally selected for CO2 absorption. The remaining infrared radiation in the band impinges upon the detector. The detector then generates an electrical response proportional in magnitude to the intensity of the infrared radiation impinging upon it which can be related to the concentration of CO2.
The reading at the detector is subject to system sensitivities which are independent of gases within the gas sample. Such system sensitivities include absorption by contaminants on the gas sample cell windows, IR dissipation due to obstructions in the radiation path, effectiveness of the manner by which the radiation is collected after passing through the sample cell, the sensitivity of the detector, and the gain of the signal processing electronics. In order to account for system sensitivities in the concentration readings, a reference detector is used. The reference detector is intended to provide a measure of the intensity of the infrared radiation in the optical path at a wavelength which is unaffected by the presence of gases likely to be in the sample cell. Therefore, the detector signal measured by the reference detector provides a measure of the basic sensitivity of the system to infrared radiation in general. That is, it provides a measure of the strength of the radiation of the IR source, the attenuation of the radiation by (non-spectral) contamination and the like on the infrared transparent windows in the sample cell, and further provides a measure of the sensitivity of the detectors as well as the gain of the processing electronics.
The band width and the center of the band for the reference detector are selected to be in an inifrared non-absorptive region for typical gas samples to be tested. Otherwise, the reading by the reference detector would be influenced by the concentration of any gases in the gas sample which absorb IR radiation before it reaches the reference detector. Therefore, it is important that the reference detector detect IR radiation at a wavelength which is displaced from the absorption wavelengths of the gases likely to be present in the gas sample.
However, it is also important that the characteristic absorption wavelength at which the reference detector detects IR radiation not be widely spaced from the characteristic absorption wavelengths of the gases of interest. This is because some system sensitivities are highly dependent on the wavelength of IR radiation used. Therefore, it is preferable to use a reference wavelength that is close to, but does not overlap, the characteristic absorption wavelength of any of the gases of interest, to increase the likelihood that the reference is a true reference which is unaffected by the concentration of the gases in the gas sample.
The output signals generated by the detectors are sent to a signal processor. The signals are ratioed to eliminate errors in the measured concentration of the gas of interest. These errors are attributable to such factors as foreign substances (e.g., condensation on the sample cell windows) and other instabilities in the infrared source and/or the detectors. A gas analyzer may also employ additional optical components, such as beam splitters, lens configurations, and the like, to increase the sensitivity and accuracy of the detectors. For example, a dichroic beam splitter may be incorporated in the beam path ahead of the detectors.
Thermopile detectors, i.e., a detector comprised of a plurality of interconnected thermocouples, are commonly used for detecting infrared radiation. The plurality of thermocouples develop a voltage output in response to impinging infrared radiation. Thermopile detectors, however, suffer from so called xe2x80x9cdriftxe2x80x9d which results in a slow variation in the voltage output of the detectors as a consequence of variations in ambient temperature and stray radiation. Thus, the measurement of gas concentrations as an absolute or steady state DC output may be difficult because of the thermal drift issue.
Means of addressing thermal drift in thermopile detectors include modulating or chopping the incident infrared beam, either by shutter type devices or by modulating the output of the infrared source of energy. Simpler systems have been designed which involve totally blocking one of the thermopile detectors and using the output thereof as a zeroing signal for comparison with the output of the other detector or detectors. Heretofore, such attempts have produced error signals due to uneven heating of the substrate. Partially attenuating the signal falling upon the zeroing detector has been suggested, but configurations taught by the prior art in which such an expedient is employed have demonstrable drawbacks.
Accordingly, it is an object of the present invention to provide an improved infrared spectrophotometer.
Another object of the invention is to provide an improved infrared spectrophotometer providing a steady state DC signal representative of gas concentration from detectors corresponding to gases of interest.
Another object of the invention is to provide an improved infrared spectrophotometer and a detector assembly therefore which effect substantial cost saving over that provided by prior art configurations.